U.S. Pat. No. Re 28,990, which is incorporated herein by reference, discloses a bipolar fetal electrode assembly commonly used to monitor fetal heart rate during birth. When using such an assembly, a physician inserts the forward end of a curved guide tube through the mother's vagina and cervix until the forward end of the guide tube makes contact with the fetal head or other portion of the fetus. Holding the forward end of the guide tube stationary, the physician then pushes the rear end of a flexible driving tube forwardly until a spiral fetal electrode at the forward end of one wire of a twisted pair of wires makes contact with the fetal epidermis. The forward end of the other wire has a spade-like reference electrode which is electrically isolated from the spiral fetal electrode.
The physician then rotates the flexible driving tube clockwise about one full turn while maintaining the forward end of the guide tube against the fetal head. This causes the spiral electrode to screw into the fetal epidermis. Thereafter, the physician removes his fingers from the mother's vagina, grasps the outer ends of the driving tube and the guide tube, and slides these tubes as a unit off the wires, leaving only the electrodes and the two twisted wires within the mother. The wires are then connected to a fetal monitor (see, for example, U.S. Pat. No. 5,199,432).
Monitoring of fetal heart rate trends through the use of an EKG electrode has long been used to indicate fetal well-being during labor and delivery. By using heart rate trends derived from the fetal EKG signal, the physician is able to infer the adequacy of oxygenation in the fetus. However, this technique is indirect and thus unsatisfactory since it is only after oxygen starvation has occurred for some time that it is reflected in the heart rate record. In addition, the heart rate record itself is subject to errors due to uterine contractions and other artifacts.
To directly monitor blood oxygenation, pulse oximeters are used. Pulse oximeters monitor blood oxygen content by measuring the absorption of light in an arterialized vascular bed. Since oxyhemoglobin and deoxyhemoglobin absorb light differently, the relative concentration of each blood component and thus the percentage oxygen saturation (SpO.sub.2) can be determined by measuring absorbed light at two different wavelengths. This method of pulse oximetry is now an established standard of care during anesthesia and in neonatal and adult critical care.
The basic design of any pulse oximeter probe contains red and infra-red light emitting diodes (LEDs) and a photodetector. These components are arranged so that the LEDs illuminate a particular section of arterialized tissue and the detector collects the light from the LEDs which has been transmitted through the tissue section but not absorbed by the skin, bone, blood and other physiologic absorbers. The steady state and time varying components of this signal are then used to calculate the fraction of the arterial blood which is oxygenated.
A vital aspect of a successful design is that the light which is received at the detector must have come only from within the tissue section being illuminated and not directly from the LEDs without having been attenuated in any way by the tissue section.
In pulse oximeter probes intended for use on adults or neonates, this may easily be arranged by utilizing a probe configuration which permits the LED emitters and the detector to be positioned on opposite sides of a section of tissue, for example a finger, toe or ear lobe on an adult or the foot of a neonate.
However, in the case of a presenting fetus where only a small section of the head is accessible, this approach is not viable, and instead reflection pulse oximetry must be employed. In this scheme, the LEDs and photodetector are placed on the same tissue surface and the photodetector receives light which has been scattered from blood vessels within the tissue section. By using reflection pulse oximetry, the potential for errors due to light being transmitted directly from the LEDs to the detector is obviously increased.
Over the years, various devices have been developed which have taken the spiral electrode concept and used the helical spring to attach an additional physiological monitoring device to the scalp, for measuring pH, PO.sub.2, or some other metabolic activity.
This approach has potential for fetal reflection oximetry since a miniature optoelectronic hybrid circuit consisting of red and infra-red LEDs and a photodetector could be constructed and placed at the end of the plastic body of the spiral electrode within the coil of the spiral. However, there exist two problems with this concept.
First, in clinical use, a spiral electrode can be attached to the scalp in such a way that a gap exists between the end of the spiral electrode body and the scalp. This has no impact on the efficacy of the spiral as an EKG electrode; however, this gap permits light from the LEDs to travel directly to the photodetector without penetrating the fetal scalp tissue. Thus, oximetry calculations based upon the received signals will not be consistent.
Second, since the LEDs and the photodetector are in close proximity, light must still be prevented from traveling directly between them even when the helical spring has been fully inserted into the fetal scalp and the distal end of the electrode holder is in intimate contact with the skin.
The above problems have been partially addressed by various prior art devices. For example, to eliminate the potential for light directly coupling between the LEDs and the photodetector, prior art devices propose that either the LEDs, or the photodetector, or both be placed below the surface of the fetal skin by means of fiber-optic light guides.
Skin penetration by the fiber-optic light guide ensures that the optical path is confined to perfused tissue. However, these approaches result in at least one additional undesirable puncture in the fetal skin.
It is therefore an object of the present invention to provide a fetal pulse oximetry sensor wherein the sensor is always in intimate contact with the fetal skin, in order to prevent light from travelling directly from the LEDs to the photodetector, with minimal puncturing of the fetal tissue.
It is also an object of the present invention to provide a fetal pulse oximetry sensor wherein the light from the LEDs is prevented from travelling directly to the photodetector when the sensor is in intimate contact with the skin.
It is a further object of the present invention to provide a fetal pulse oximetry sensor that does not cause additional trauma to the fetal skin, viz., punctures.
It is also an object of the present invention to provide a fetal electrode product that includes a fetal heart rate electrode and a pulse oximetry sensor.